Calculating potential between two points + additional questions

Discussion in 'Homework Help' started by ZeroTorrent, Mar 28, 2012.

  1. ZeroTorrent

    Thread Starter New Member

    Mar 28, 2012
    10
    0
    Hello all! This is my first time posting on your lovely forums!

    I am something of a complete idiot when it comes to circuit theory. I have done many university courses in advanced Quantum mechanics and math but circuit theory makes me want to curl up in the shower and cry :p

    There seems to be a particular type of problem that stumps me and it makes me rather confused.

    In problem 1 you can see a typical circuit of which i'm supposed to make a Thevinin circuit over R4. I use the node method as that method works (most of the time) and it's fairly easy. I take the bottom rectangle and set it as grounded and the other one as node e. The problem is that I am not sure what to do with E3 using the node method.

    Node e: (current going in = current going out)

    i_1 + i_2 = i_3 => (E1-e)/R1 + (E2-e)/R2 - e/R3 = 0 => E1/R1 +E2/R2 = e(1/R1+1/R2+1/R3) => inserting numbers => 0.4 = 0.05e => e = 8 Volts

    I really wish I knew how to incorporate E3 in to my node equation. This is also where I get confused; Looking at a solution i've done everything right except I need to subtract the 6V of E3, so U_AB = A - 6 = 2 V. I thought it would add because of the polarity of E3: It flows perfectly in to node A and should this make the potential higher. There are other problems where I thought I'd get an addative potential when one should actually be subtracted from the other. Could someone sort out this problem and explain it for me?
     
  2. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
    1,097
    Can you post a circuit diagram?
     
  3. ZeroTorrent

    Thread Starter New Member

    Mar 28, 2012
    10
    0
    Ofc, sorry :p
     
  4. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,957
    1,097
    You answer looks good to me.
    Ve = 8V
    Because positive side of the E3 voltage source is point toward Ve.
    This means that "+" terminal of the E3 has a higher potential (higher by 6V) than negative terminal.
    And this is why Ve = Va + E3 must be true.
     
    Last edited: Mar 28, 2012
    ZeroTorrent likes this.
  5. ZeroTorrent

    Thread Starter New Member

    Mar 28, 2012
    10
    0
    Okay, I think I understand now. After using the node calculations I can fall back on KVL over node e. It's just an additinal step where as I do calculate using all currents don't get all voltages and have to account for them seperatly.

    Thank you! :)
     
Loading...